1. Field of the Invention
The present invention relates to an improved device for measuring physical characteristics of a more or less porous sample. Such a device is notably well suited to test geologic samples and to determine various parameters such as the capillary pressure of rocks in drainage and imbibition phases, their wettability index, their relative permeability, their resistivity index, etc.
2. Description of the Prior Art
It is important to determine the wettability of rocks with respect to the water and to the oil which may be contained therein. To that effect, the rock must be subjected to a drainage process, that is a displacement of the fluids intended to decrease the water saturation thereof, followed by an imbibition, that is a displacement of the fluids allowing an increase of the water saturation (Sw) of the rock. The capillary pressure at one point is defined as the difference Pc at equilibrium between the pressure Po of the oil and the pressure Pw of the water. This parameter is meaningful only if the two fluids are in the continuous phase in the porous medium. For a water wet medium, only positive values are meaningful. On the other hand, if the medium has a mixed wettability, the fluids can remain in the continuous phase for the positive and for the negative capillary pressures (Pc) as well.
For an application of this type, a complete capillary pressure measuring cycle must therefore comprise (FIG. 1):
a) positive primary drainage of an initially 100% water-saturated sample (curve 1),
b) positive imbibition (curve 2),
c) negative imbibition (curve 3),
d) negative drainage (curve 4), and
e) positive secondary drainage (curve 5).
Knowledge of various parameters and notably of the wettability of rocks is useful notably when enhanced recovery is to be performed in a formation, by draining the effluents contained therein by injecting a fluid under pressure, and when the most suitable fluid (water or gas) for displacement of effluents is to be determined by means of preliminary tests.
French Patent A-0,603,040 filed by the assignee describes a method allowing measurement of physical characteristics of saturated rocks by subjecting them to a progressive-speed centrifugation and by measuring the amount of fluid displaced as a function of the rotating speed. The sample saturated with a liquid A for example is placed in an elongate container or vessel containing another fluid B of different density. The vessel is fastened to the end of a rotating arm and a centrifugal force is applied thereto so as to study the fluid displacements in the sample during at least two distinct phases. During a first drainage phase, the assembly is subjected to a centrifugal force applied along the length of the vessel so as to exert an expulsion force thereon, which tends to flow out of part of the first fluid B. At the same time, some of fluid A flows into the sample. The two fluids move through the sample until they reach a position of equilibrium where the force due to the capillary pressure in the pores compensates for the centrifugal force exerted.
It is well-known that the capillary pressure PC at a distance R from the fulcrum pin, when it is positive, is expressed by the following relation:PC(R)=½Δρω2(R2max−R2)PC(Rmax)=0where ω is the angular rotating speed, Rmax is the distance from the base of the sample bar S to the fulcrum pin, and Δρ is the difference between the respective densities of the two fluids.
For negative values, the capillary pressure PC at a distance R from the fulcrum pin is:PC(R)=½Δρω2(R2min−R2)PC(Rmin)=0.
During the re-imbibition phase (curve 2), the speed is decreased in order to study the re-integration of the initial fluid therein. With this type of method, local saturations are calculated by an inversion program from the total amount of water expelled from the sample.
The capillary pressure in the sample can be deduced from the precise measurement of the amount of initial fluid extracted as a function of the centrifugal force exerted and from the variation of the average fluid saturation Sm of the sample as a function of the centrifugal force exerted, which is obtained, for example, by acoustic detection.
With a fluid-saturated sample, it can be seen (FIG. 1) that the saturation during the centrifugal drainage phase, for a determined radius r, decreases (curve 1) as the rotating speed w increases until a minimum value Si is reached. During this drainage phase, the rotating speed is increased in successive stages until a speed of 3500 rpm is reached for example. The fluid saturation variations are measured during the deceleration phase. A hysteresis phenomenon and a return, according to another variation curve (curve 2), to a relative maximum value Sm are observed during the re-imbibition phase of the porous material.
A system for maintaining the drained fluid in contact with the sample bar is preferably used so that, when the deceleration phase starts, the bar can be properly re-imbibed. To ensure this maintenance, the system stabilizes the interface level between the two fluids at a minimum level where it is level with the base of the bar, that is at the furthest distance from the fulcrum pin (Rmax), at least throughout the deceleration phase.
Displacements inside the sample are followed either by measuring the variation of the time of flight of the ultrasounds through the sample, or by measuring the variation of the electric resistance thereof. The volume drained can be measured optically by a vessel provided with a transparent light and by observing the level variation by use of a stroboscopic lighting.
The fluids drained out of the sample can for example be transferred into a variable-volume chamber in the same vessel or in a second rotating vessel for example, by a pump borne by one of the arms and driven by an electric motor. Such a system is easy to implement and has a reasonable cost. However, it requires using a pump borne by the arm and therefore subjected to the centrifugal force. This is a drawback because it is difficult to find standard electric driving motors capable of withstanding the great accelerations required for implementation of the process, typically of the order of 3000 g. Special motors whose cost is very high must be used therefore.